Electronics Engineering (ELEX) Board Practice Exam

What does a dim indicator on a logic probe typically suggest?

1. Excessive load applied

2. No power supply voltage

3. The output stage is damaged

4. The output signal is not present

The correct answer is: The output signal is not present

A dim indicator on a logic probe typically suggests that the output signal is not present. When using a logic probe to test digital circuits, the probe indicates the presence and state of the signal being measured. If the output signal is high, the probe will illuminate brightly, whereas a dim light indicates that there is insufficient signal strength reaching the probe or that the expected logic level is not being met. In practical terms, this can often mean that the logic level is low or that there is insufficient voltage swing for the probe to register a strong, clear signal. It indicates a potential issue in the circuit where the expected output is not being produced, leading to a lack of a definitive high or low signal. This situation requires further investigation to determine why the output is missing, whether it's due to a fault elsewhere in the circuit or insufficient drive from the output stage. Understanding this behavior helps diagnose problems in digital circuits, allowing engineers to pinpoint where a malfunction may be occurring, whether it be a failed component, a disconnection, or a design issue that limits the output signal's integrity.